This invention relates to high power load pull (see ref. 4) testing of microwave transistors (DUT) in the nonlinear operation regime. When driven in its nonlinear operation regime, the internal impedance at the output port of power transistors, DUT (21) is very low, of the order of 1Ω An impedance tuner (20) used to match the transistor must also physically match such impedance (FIG. 1). Passive impedance tuners (23, 20) are used to generate such impedance. The signal (25) is injected into and extracted from the DUT (21) via bidirectional couplers (11, 10) which allow the power waves travelling along the main line to be sampled (26) and measured by the receiver (VNA, 24). The output port of tuner (20) is terminated (19) with characteristic impedance Zo (50Ω). The VNA (24) and tuners (23, 20) are controlled digitally (17, 18) by a system controller (22).
Passive tuners can create maximum reflection factors |Γtuner| of the order of 0.95, corresponding to a smallest impedance of 2.45Ω. The unavoidable insertion loss between DUT and tuner reduces even further the available tuning range at the DUT reference plane and thus the capacity of the passive tuner to match the transistor. The only remedy to this limitation is using “active” systems, i.e. test systems whereby a signal coherent (synchronous) with the signal injected and amplified by the DUT and reflected into the output port of the DUT by the passive tuner, is injected independently into the DUT and creates a partly real and partly virtual load. The DUT cannot distinguish which is which. This additional signal can be the only one injected, in which case we speak of “purely active” load pull, or it can be superimposed to signal reflected by the passive tuner, in which case we speak of “hybrid” load pull; obviously, if only a mechanical (passive) tuner is present, we speak of “passive” load pull. In both active injection cases (purely active and hybrid) the objective is reaching and conjugate matching the internal impedance of the transistor; in general terms a standard requirement is a tuning range reaching a reflection factor |Γ|=1 (corresponding to a real part of the internal impedance of 0Ω). The objective of this invention is an active tuner apparatus, combining a forward signal injection mechanism within an electro-mechanical tuner architecture allowing |Γ|=1.